1. Technical Field
The present invention relates to chairs and more particularly to a resilient-support adjusting device of a chair backrest. The resilient-support adjusting device includes a base and a spring-tensioning brace and a backrest-fastening seat pivotally connected to two ends of the base. A spring is tensioned between the spring-tensioning brace and the backrest-fastening seat. The base contains upper and lower abutting members that have inclined abutting surfaces abutting against each other. The upper abutting member has an upper end abutting against a swinging end of the spring-tensioning brace. The lower abutting member has an engaging member that can be driven by the operating lever to rotate. The engaging member and the lower abutting member engage with each other. When rotated, the operating lever drives the spring-tensioning brace to swing and draw the spring. The spring thus draws the backrest-fastening seat harder, making the backrest have a stronger backward resilient support.
2. Description of Related Art
In existing office chairs, the backrest is typically provided with a resilient-support mechanism. For example, Taiwan Patent Nos. TWI429411 (B), TWM395420 (U), TWM321261 (U) and M480321 (the same invention as claimed in China Patent No. CN201320493667) all disclose resilient-support mechanisms using a spring to draw or prop up a lower end of a swinging backrest, so that the resilient compressing force of the spring can endow the backrest with resilient support. These patents also have common operational means wherein a rotatable screw uses its threads to drive a slidable or swingable member to displace, to press or release the spring, making the spring have different levels of returning force, and in turn providing different levels of resilient support to the backrest.
While the known means achieved by the screw does support step-less adjustment of resilient support, in practical use, such step-less adjustment fails to provide a well perceptible difference. That is, by rotating the screw for two to three rounds, the pressing or releasing movement that the screw applied to the spring is too slight for a sitter sitting in the chair to feel the change in the resilient support. A difference perceptible at the backrest by the sitter can only appear when the screw is rotated for more than five rounds. It is very likely that, after repeated operation and test, the sitter has already forgotten how many rounds he/she made to the screw. When the next time he/she sits this chair, the repeated operation and test have to be performed again. This makes the adjustment highly inconvenient.
In addition, the level of resilient support of backrests is determined by the number of the rounds the screw is rotated. To make the change of the resilient support perceptible, the screw has to be rotated for many rounds. In this case, the operator has to continuously rotate the screw, making the operation effort-consuming and time-consuming. Thus, the known approach to adjustment is less capable of meeting a sitter's needs of different levels of resilient support.